noclip. Be it for trolling, cheating, or just being plain ridiculous (all three, see: Garry's Mod), noclip has limitless uses and never ceases to make a game unpredictable.
Since it has almost become an integral part of GM, if it doesn't count as a cheat, then the cheat on Gauntlet Dark Legacy that gave you infinite Pojo (fire breathing chicken). Was it that useful? No. In fact it made you the worst character on the team. But it made you a chicken, and that was enough to make my friends jealous. Besides, I can't name one game that can't be improved by being a fire breathing chicken.